This invention relates generally to aluminum alloys that can be cast into structural components; non-limiting examples of which include engine blocks, cylinder heads, suspension parts such as shock towers and control arms, wheels, and airplane doors.
Al—Si based cast aluminum alloys, such as the 300 series aluminum alloys, have widespread applications for structural components in the automotive, aerospace, and general engineering industries because of their good castability, corrosion resistance, machinability, and, particularly, high strength-to-weight ratio in the heat-treated condition. In terms of castability, low silicon concentrations have been thought to inherently produce poor castability because of the increased freezing range and the reduced latent heat. With high Si content (>14%), however, the coarse primary Si particles will significantly reduce machinability, ductility and fracture toughness of the materials.
In Al—Si casting alloys (e.g. alloys 319, 356, 390, 360, 380), strengthening is achieved through heat treatment after casting with addition of various alloying elements including, but not limited to Cu and Mg. The heat treatment of cast aluminum involves at least a mechanism described as age hardening or precipitation strengthening that involves, but is not limited to, three steps including (1) solution treatment at a relatively high temperature below the melting point of the alloy (also defined as T4), often for times exceeding 8 hours or more to dissolve its alloying (solute) elements and homogenize or modify the microstructure; (2) rapid cooling, or quenching into cold or warm liquid media such as water, to retain the solute elements in a supersaturated solid solution (SSS); and (3) artificial aging (T5) by holding the alloy for a period of time at an intermediate temperature suitable for achieving hardening or strengthening through precipitation. Solution treatment (T4) serves three main purposes: (1) dissolution of elements that will later cause age hardening, (2) spherodization of undissolved constituents, and (3) homogenization of solute concentrations in the material. Quenching after T4 solution treatment is to retain the solute elements in a supersaturated solid solution and also to create a supersaturation of vacancies that enhance the diffusion and the dispersion of precipitates. To maximize strength of the alloy, the precipitation of all strengthening phases should be prevented during quenching. Aging (T5, either natural or artificial aging) creates a controlled dispersion of strengthening precipitates.
The most common Al—Si based alloy used in making automotive engine blocks and cylinder heads is heat treatable cast aluminum alloy 319 (nominal composition by weight: 6.5% Si, 0.5% Fe, 0.3% Mn, 3.5% Cu, 0.4% Mg, 1.0% Zn, 0.15% Ti and balance Al) and A356 (nominal composition by weight: 7.0% Si, 0.1% Fe, 0.01% Mn, 0.05% Cu, 0.3% Mg, 0.05% Zn, 0.15% Ti, and balance Al). Because of the relatively low Si content (6˜7 wt %) in both alloys, the liquidus temperatures are high (˜615 C for A356 and ˜608 C for 319) leading to a high melting energy usage and high solubility of hydrogen. The high freezing range of both A356 (greater than or equal to 60 C) and 319 (greater than or equal to 90 C) also increases the mushy zone size and shrinkage tendency. Importantly, both alloys present dual microstructures of primary dendritic aluminum grains and eutectic (Al+Si) grains. During solidification, the eutectic grains solidify between the pre-solidified dendritic Al networks which makes feeding eutectic shrinkage difficult. In Al-7% Si alloys, the volume fraction of eutectic grains is about 50%. In addition, the engine blocks and particularly cylinder heads made of such aluminum alloys may experience thermal mechanical fatigue (TMF) over time in service, especially in high performance engine applications.
The addition of strengthening elements such as Cu, Mg, and Mn can have a significant effect on the physical properties of the materials, including specific undesirable effects. For example, it has been reported that aluminum alloys with high content of copper (3-4%) have experienced an unacceptable rate of corrosion especially in salt-containing environments. Typical high pressure die casting (HPDC) aluminum alloys, such as A 380 or 383 used for transmission and engine parts contain 2-4% copper. It can be anticipated that the corrosion issue of these alloys will become more significant particularly when longer warranty time and higher vehicle mileages are required.
Although there is a commercial alloy 360 (nominal composition by weight: 9.5% Si, 1.3% Fe, 0.3% Mn, 0.5% Cu, 0.5% Mg, 0.5% Ni, 0.5% Zn, 0.15% Sn and balance Al) designated for corrosion resistance applications, such alloy may experience thermal mechanical fatigue problems over time in service, especially in the high performance engine applications.
There is a need to provide improved castable aluminum alloys that are suitable for both sand and metal mold casting and can produce castings with reduced casting porosity and improved alloy strength, fatigue, and corrosion resistance, particularly for applications at elevated temperatures.